1. Field of the Invention
The present invention relates to an image forming apparatus having charging means for charging an image bearing body or image bearing body. It is preferred that the present invention is applied to a cleanerless image forming apparatus capable of removing and recovering developer (toner) remaining on the image bearing body after the transfer process so that the recovered developer will be recycled by a developing device in the process of simultaneous development and cleaning without the need for a cleaning device.
2. Related Background Art
A conventional transfer type image forming apparatus, such as a copier, a printer and a facsimile, using a transfer type electrophotographic method is constituted of a photosensitive body, a charging device (charging process), an exposing device (exposing process), a developing device (developing process), a transfer device (transfer process), a cleaning device (cleaning process), a fixing device (fixing process), and so on. The photosensitive body is an image bearing body or image bearing body typically formed in the shape of a rotary drum. The charging device (charging process) statically charges the surface of the photosensitive body uniformly to predetermined polarity and potential. The exposing device (exposing process) writes information for forming an electrostatic latent image on the charged surface of the photosensitive body. The developing device (developing process) develops the electrostatic latent image formed on the photosensitive body with toner as developer to make the latent image visible. The transfer device (transfer process) transfers the toner image from the surface of the photosensitive body to a transfer material such as a paper sheet. The cleaning device (cleaning process) cleans the surface of the photosensitive body by removing toner slightly remaining on the photosensitive body after the transfer process. The fixing device (fixing process) fixes the toner image on the transfer material. In this arrangement, the photosensitive body is repeatedly used in the above-mentioned electrophotographic processes (charging, exposing, developing, transfer and cleaning) to form images.
The toner remaining on the photosensitive body after the transfer process is removed by the cleaning device from the surface of the photosensitive body and stored in the cleaning device as waste toner, but it is desirable not to create such waste toner from environmental protection and resource saving standpoints.
Therefore, there have been provided image forming apparatuses capable of returning, to the developing device, residual toner after transfer, that is, waste toner collected by the cleaning device so that the collected toner will be recycled.
Cleanerless image forming apparatuses have also been provided, in which the toner remaining on the photosensitive body after the transfer process is removed from the photosensitive body and recovered into the developing device in the process of xe2x80x9csimultaneous development and cleaningxe2x80x9d so that the recovered toner will be recycled by the developing device.
In the process of simultaneous development and cleaning, the toner remaining on the photosensitive body after transfer is recovered into the developing device for subsequent cycles of development. In other words, residual toner existing on a surface portion (non-image portion), not to be developed with toner, of the photosensitive body is recovered into the developing device by applying a bias for eliminating developing fog in the process of developing an electrostatic latent image for the next cycle of electrophotographic processes after continuously charging the photosensitive body, exposing the same to form the next electrostatic latent image. The bias Vback for eliminating developing fog denotes a difference in potential between voltage applied to the developing device and surface potential of the photosensitive body. The use of this process allows the toner remaining after transfer to be recovered into the developing device so that the recovered toner will be recycled for subsequent cycles of development of electrostatic latent images. Therefore, waste toner is eliminated, and hence troublesome maintenance work can be reduced. Further, since this type of image forming apparatus is cleanerless, it also has the advantage of reducing the overall apparatus size.
a) The above-mentioned type of cleanerless image forming apparatus removes the residual toner after the transfer process from the photosensitive body and recovers the removed toner into the developing device in the process of simultaneous development and cleaning so that the recovered toner will be recycled, but it has the following drawbacks. Suppose that the cleanerless image forming apparatus uses a contact type charging device which comes in contact with the photosensitive body to statically charge the surface of the photosensitive body. In this case, when the toner remaining on the photosensitive body after transfer passes through a charging portion as a contact nip portion between the photosensitive body and the contact charging device, some of the residual toner, especially toner that is charged to an opposite polarity to a normal polarity, tend to stick to the contact charging device, that is, to contaminate the contact charging device with toner inadmissibly, failing to charge properly.
Toner as developer naturally contains toner particles of the opposite or reverse polarity to the normal charging polarity, though they are limited in quantity. Even the toner particles charged to the normal polarity may reverse its polarity under the influence of a transfer bias or peeling discharge, or may be statically erased to reduce its charging amount.
Thus, the toner particles of the normal polarity, the toner particles of the reverse polarity, and the toner particles having a small charging amount are mixed up in the residual toner after transfer. Of all the mixed-up toner particles, the reversely charged toner particles and the toner particles having a small charging amount tend to stick to the contact charging device when passing through the contact nip portion in which the contact charging device comes in contact with the photosensitive body.
b) The following conditions are required for the developing device to remove and recover the toner remaining on the photosensitive body after transfer in the process of simultaneous development and cleaning. One condition is that the toner remaining on the photosensitive body after transfer and carried to the developing portion after passing through the charging portion must be statically charged to the normal polarity. The other condition is that an adequate charging amount of the toner to enable the developing device to properly develop the electrostatic latent image on the photosensitive body is needed. The reversely charged toner or the toner inadequate in the charging amount may not be removed and recovered from the photosensitive body, resulting in the occurrence of an image defect.
c) The adhesion of toner to the contact charging device mentioned in the foregoing paragraph a) can be prevented by taking the following measures. In other words, the adhesion of toner can be prevented by toner-charging amount control means directing all the toner particles remaining on the photosensitive body after transfer to the normal polarity while making the charging amount uniform. Although the toner remaining on the photosensitive body after transfer is carried from the transfer portion to the charging portion in the form of a mixture of normally charged toner, reversely charged toner and toner having the small charging amount, the application of a normal-polarity charge to the residual toner directs the residual toner all to the normal polarity.
However, the residual toner applied with the normal-polarity charge by the toner charging amount control means for the purpose of preventing the toner adhesion to the contact charging device becomes too large in its charging amount to properly develop the electrostatic latent image on the photosensitive body. This makes it difficult for the developing device to remove and recover the residual toner in the process of simultaneous development and cleaning. In this case, the next image is superimposed on the toner remaining on the photosensitive body to cause the occurrence of an image defect.
In addition, recent demands for continuous printing of images with high print quality such as of photographic images accompanied by diverse user needs, and a multiple development system for color printing involve occurring a large amount of toner remaining after transfer at a time, further promoting the above-mentioned drawbacks.
It is an object of the present invention to provide an image forming apparatus capable of properly correcting the charging polarity and charging amount of developer remaining on an image bearing body.
It is another object of the present invention to provide an image forming apparatus capable of preventing the developer remaining on the image bearing body from causing the occurrence of defects in subsequent images.
It is still another object of the present invention to provide an image forming apparatus capable of preventing the developer remaining on the image bearing body from sticking to charging means.
It is yet another object of the present invention to provide an image forming apparatus capable of efficiently recovering the developer remaining on the image bearing body into developing means.
It is yet another object of the present invention to provide an image forming apparatus suitable for a cleanerless system.
Other objects and features of the present invention will become further apparent from reading the following detailed description of embodiments with reference to the accompanying drawings.